Unlike a typical hydraulic system having a single pump feeding a plurality of solenoid valves to control an associated plurality of functions, a “meterless” hydraulic control system controls each hydraulic actuator of each function by controlling a flow rate from a dedicated pump associated with that actuator. Thus, while proportional or throttling valves are utilized in prior art metered systems to meter fluid to control movement of each actuator, the flow to each actuator in a meterless system is controlled directly by controlling the associated pump. The dedicated pump or pumps may be of any suitable type including variable displacement or fixed displacement, wherein the flow from the pump to the actuator chambers is varied in order to control the speed and extent of the actuator movement.
In prior art meterless arrangements, pump controlled circuits known as Displacement Controls (DC) utilize a variable displacement pump with a constant speed driver, while Electro-Hydrostatic Actuators (EHA) utilize a fixed displacement pump with a variable speed driver. In either case, since actuator flow is controlled by the pump, the hydraulic circuit associated with one or more actuators may experience and overpressure condition when the associated actuated element encounters an obstruction. Typical practice is to provide a relief valve through which fluid is vented to relive the excess pressure. In this arrangement, whenever the set release pressure of the valve is reached, the valve opens and the pressure decreases. When the pressure has decreased to below the valve limit, the valve shuts again.
Although this type of system allows for pressure control, it does so at the expense of fuel efficiency and system. In particular, the release of hydraulic fluid to lower pressure wastes the energy stored in the fluid at that point.